PARIS, March 16 (Reuters) - Bernard Arnault, 73, could extend his tenure at the helm of luxury house LVMH (LVMH.PA) to the age of 80, according to a company filing ahead of its annual shareholder meeting. Shareholders of the world’s largest luxury group, which owns dozens of high end labels including Louis Vuitton, Dior and Tiffany, will vote on April 21 on ...